Torque of turning wheel

1. Jan 28, 2009

nns91

1. The problem statement, all variables and given/known data
1.
A wheel mounted on an axis that is not frictionless is initially at rest. A constant external torque of 50 N.m is applied to the wheel for 20s, giving the wheel an angular velocity of 600 rev/min. The external torque is then removed, and the wheel comes to rest 120 s later. Find (a) the moment of inertia of the wheel, and (b) the frictional torque, which is assumed to be constant.

2. A wagon wheel 1.0m in diameter consists of a thin rim having a mass of 8kg and six spokes each having a mass of 1.2kg. Determine the moment of intertia of the wagon wheel for rotation about its axis.

2. Relevant equations

$$\tau$$ = I*$$\alpha$$

3. The attempt at a solution

1. I found out the moment of inertia by subsituting 50 for $$\tau$$ and calculat the angular acceleration by change in velocity / change in time.

How do I do part b ?? I am kind lost and don't know what approach to use.

2.So how do I approach this problem ? Do the spkes matter ?? Do I need an integration here ??

2. Jan 28, 2009

Staff: Mentor

Re: Rotation

Not sure your approach to (1) is correct. 50Nm is not the net accelerating torque... You should write two equations for the behavior of the wheel, and use them to solve for the two unknowns....

3. Jan 28, 2009

nns91

Re: Rotation

What are those two equations then ?

4. Jan 28, 2009

Staff: Mentor

Re: Rotation

You tell us. You know how the Homework Help forums work....

5. Jan 28, 2009

nns91

Re: Rotation

I guess it will base on the T=I*a (I mean T as torque and a as alpha).

I can calculate alpha so I still have T and I left.

So I know the external torque is 50 but there is friction also but I don't know friction yet.

Can you give me like a hint ??

6. Jan 28, 2009

Staff: Mentor

Re: Rotation

Hint -- when the wheel is spinning up, how many torques are acting on it? When it is spinning down, how many torques are acting on it?

7. Jan 28, 2009

nns91

Re: Rotation

When it spins up, there is one at the top, when it spins down, there is one at the bottom.

Am I right ??

8. Jan 28, 2009

Staff: Mentor

Re: Rotation

That's not the way I'm reading the problem (but I could be wrong I guess). It sounds like there is axle friction (not sure why it's called axis friction) torque. Friction always retards motion, right?

9. Jan 28, 2009

nns91

Re: Rotation

yep. So Torque= 50- Friction torque right ?? However, I don't know friction torque.

10. Jan 28, 2009

Staff: Mentor

Re: Rotation

Two equations and two unknowns...

11. Jan 28, 2009

nns91

Re: Rotation

So the first one will be: 50- Friction = alpha * I ??

12. Jan 28, 2009

Staff: Mentor

Re: Rotation

Just write 'em both down and solve 'em.

13. Jan 28, 2009

nns91

Re: Rotation

A dumb question but what do you mean by both ??

14. Jan 28, 2009

Staff: Mentor

Re: Rotation

Two equations and two unknowns.... Write down both equations and solve away.

I have to bail in a couple minutes. Think it through.

15. Jan 28, 2009

nns91

Re: Rotation

I thought I have only one equation ??

16. Jan 28, 2009

Staff: Mentor

Re: Rotation

I'm not going to hold your hand every step of the way. Go back and read all the posts in this thread, and solve the problem. Honestly, it's all there. And check your PMs in a couple minutes.

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